The invention relates generally to optics, and more particularly to a pivoting element for optical applications.
Optical switches have numerous applications for optical networks in areas such as cross-connects. Micro-electro-mechanical system (MEMS) optical mirrors have been developed for use in such switches. MEMS devices are fabricated using photolithographic techniques similar to those developed for mass production of semiconductor integrated circuits.
In a conventional MEMS optical switch, as seen for example in U.S. Pat. No. 6,044,705, electrostatic forces are used to pivot a suspended mirror in a desired direction. In this manner, the mirror may direct light beams at a desired angle. For accurate optical switching, however, the mirror should be pivoted very precisely so that the desired angle(s) may be achieved repeatedly with high precision. The resulting angle is determined by the balance between the applied electrostatic force and the spring restoring force applied by the mirror""s suspension. This balance drifts with variations both in temperature and in stress. To prevent the drift from affecting desired results, complicated feedback circuitry is often necessary to control the direction of the mirror.
To address the need for accurate and repeatable positioning of the mirror for optical switching, U.S. Pat. No. 6,212,309 discloses a suspended rectangular mirror that pivots on its axis in the familiar manner of a playground seesaw. Just as a seesaw""s pivoting motions is stopped in the same position when it contacts the ground, the disclosed rectangular mirror will come to rest at the same angle when it is fully deflected against its substrate. This mirror also suffers from certain disadvantages. For example, it has only two fully deflected positions, and is thus limited in the number of angles in which it can direct light when in the fully deflected position.
Copending U.S. patent application Ser. No. 09/870,967, entitled xe2x80x9cSnap-Down Pivoting Optical Element,xe2x80x9d discloses a snap-down mirror supported by a pillar, where that mirror tilts on the upper surface of that pillar. The pillar extends upward from a platform, which in turn rests on a substrate. The platform is polygonal, having a perimeter composed of a number of micromachined linear segments. When the mirror is fully deflected, it contacts a linear segment at the edge of the platform. In this way, the mirror comes to rest in a fully deflected position in a plane defined by the linear segment of the platform contacted by the mirror and the point on the upper surface of the pillar contacting the mirror. The snap-down mirror can deflect light in a number of different angles, each corresponding to a different linear segment of the perimeter of the platform.
For snap-down mirrors as described above, the angle of the mirror in the snap-down position is defined by the pivot point of the mirror and a contact line between the mirror and the substrate. That is, the plane of the mirror is defined by the center pivot point and a linear segment along which the mirror and the substrate contact one another. The contact line may extend across a substantially linear edge of the mirror that contacts the surface of the substrate, or across a substantially linear edge of the substrate that contacts the underside of the mirror. The accuracy with which the snap-down mirror can be positioned is limited by the accuracy with which the contact surfaces are micromachined. However, even with accurate micromachining, the edges of the mirror and the surface of the substrate are rough at a microscopic level. Microscopic ridges, voids and other defects may be present on the mirror, substrate or both at the point of contact between the two. As a result, the contact between the mirror and the substrate can result in inaccurate positioning between them. For example, microscopic ridge defects along the line of contact between the mirror edge and the surface of the substrate result in contact occurring at that ridge, rather than along the surface of the substrate. As a result, the mirror does not snap down to the expected position, because the ridge defect prematurely stops the deflection of the mirror. Further, the surfaces of the mirror and/or substrate may wear down after numerous contacts between them, thereby reducing the repeatability of the mirror position. For example, a defect present on the substrate may snap off after months of operation of the snap-down mirror, such that the angle of the mirror in the snapped-down position changes.
Further, fine adjustments to the angle of the mirror are not possible, because the angle of the mirror after deflection is purely determined by the pivot point of the mirror and the points of contact between the mirror and the platform. Thus, the mirror cannot be adjusted to compensate for slight deviations of mirror angle that may result from, among other sources, micromachining defects on the mirror.
Additionally, snap-down mirrors as described above may be vulnerable to interruption in operations as a result of particulate matter or debris entering the vicinity of the mirror. Such debris can drift between the mirror and the substrate, such that the mirror snaps down onto the debris instead of the substrate. When this happens, the mirror does not snap down to its expected position, and a switching error may occur. Because contact between the mirror and the substrate takes place along a contact line, the presence of debris anywhere along that contact line can result in a switching error.
Finally, snap-down mirrors as described above are designed to overcome significant stiction effects resulting from contact between the mirror and the substrate. Stiction is the unintentional adhesion of MEMS surfaces, and can result from one or more factors such as surface tension, electrostatic forces, van der Waals forces, humidity-driven capillary forces, and other factors. The stiction force between two MEMS surfaces is the force required to separate the two surfaces after they are brought into contact with each other, and this force increases as the area of contact between the two surfaces increases. Thus, the substantially linear contact area between the mirror and the substrate may be large enough to result in substantial stiction force that cannot be overcome easily, if at all.
Accordingly, there is a need in the art for a MEMS pivoting element that can accurately and repeatedly position itself in a plurality of directions.
A plurality of kinematic supports in conjunction with a snap-down mirror and n underlying platform allow for control of the mirror angle.
In one aspect of the invention, a snap-down mirror is supported on a pivot point that extends upward from a platform, where the platform is placed on a baseplate. One or more electrodes are located on the upper surface of the platform, and at least one kinematic support is placed on each electrode. The kinematic supports are nonconductive, to prevent grounding through them. The kinematic supports are photolithographically defined or otherwise constructed. One or more corresponding kinematic supports may be provided on the upper surface of the pivot point. When the mirror is snapped down, two kinematic supports on the platform and a kinematic support on the pivot point contact the mirror, thereby forming a plane defined by the points of contact between the mirror and the three kinematic supports. Thus, the manufacturing tolerances of the linear segments of the platform can be relaxed, and the angles defined between the mirror and the substrate can be controlled more accurately.
In another aspect of the invention, the kinematic supports project from the platform, separate from the electrodes. The kinematic supports can be electrically biased to a different potential than the electrodes. In this way, a higher voltage can be applied to the electrodes without electrical breakdown from an electrode to the mirror through a kinematic support.
In another aspect of the invention, a plurality of kinematic supports are placed on the snap-down mirror, instead of on the electrodes, platform or pivot point. When the mirror is snapped down, the kinematic supports on the mirror contact the platform and/or an electrode on the platform, as well as the pivot point, thereby forming a plane defined by the three kinematic supports.
In another aspect of the invention, a compliant flexure is provided at one or more points on the platform. Each flexure is provided under a kinematic support, and allows for a degree of compression. Thus, fine changes in the angle of the mirror may be made by deflecting the mirror, then exerting a small additional amount of force on the mirror, as by changing the voltage on one or more electrodes on the substrate. In this way, the flexures allow the mirror to move in response to the additional force, such that fine adjustment can be provided as needed.
In another aspect of the invention, one or more kinematic supports may be moved a selected amount by the application of an actuation force, without the use of flexures. Such an actuation force may be a piezoelectric force, generated by applying voltage to a piezoelectric layer placed underneath a kinematic support. In this way, actuation force is applied to the mirror when it is snapped down onto that kinematic support.
In another aspect of the invention, the platform is sloped downward from the central pillar, where the platform includes one or more discrete facets. One or more electrodes are located on each facet. By providing a sloped platform, the distance between the snap-down mirror and the platform is reduced, thereby reducing the voltage required to snap down the mirror.